This disclosure generally relates to digital color printing machines, such as printers, copiers and scanners and specifically relates to improving color printing and enhancing paper handling.
Color images are typically produced by the well-known process of electrophotographic or xerographic printing. In electrophotographic printing, a charge retentive surface, typically known as a photoreceptor, is charged, and then exposed to a light pattern to selectively discharge the surface according to a desired image. The resulting pattern of charged and discharged areas on the photoreceptor forms an electrostatic charge pattern, known as a latent image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as toner. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. After the toner image is produced in conformity with the light image of the desired image, the toner image may then be transferred to a substrate and then affixed (fused) to form a permanent image on the substrate. The charge retentive surface is then cleaned to prepare for subsequent development.
In the process of electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as development. In the development process, there are two commonly used development materials: single-component developer and two-component developer. Single-component developer consists entirely of toner, while two-component developer consists of toner particles and carrier beads. One type of toner is emulsion aggregation (EA) toner, which is characterized by it's spherical shape.
In two-component developer material, the toner particles are triboelectrically adhered to the carrier beads. When the developer material is placed in a magnetic field, the toner particles adhered to the carrier beads form what is known as a magnetic brush. The carrier beads form chains that resemble the fibers of a brush. This magnetic brush is typically created by a developer roll. One type of development that uses a magnetic brush is semi-conductive magnetic brush development (SCMB). Examples of other development systems include hybrid scavengeless development, hybrid jumping development and standard magnetic development. The developer roll is typically a cylindrical sleeve rotating around a fixed assembly of magnets. The carrier beads form chains extending from the surface of the developer roll. The toner particles are electrostatically attracted to the chains of the carrier beads. When the magnetic brash is introduced into a development zone adjacent to the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor causes the toner particles to be pulled off the carrier beads and onto the photoreceptor.
In single-component developer material, each toner particle has both an electrostatic charge to enable the particles to adhere to the photoreceptor and magnetic properties to allow the particles to be magnetically conveyed to the photoreceptor. Instead of using magnetic carrier beads to form a magnetic brush, the magnetized toner particles adhere directly to a developer roll. In the development zone adjacent to the electrostatic latent image on the photoreceptor, the electrostatic charge on the photoreceptor causes the toner particles to be attracted from the developer roll to the photoreceptor.
A variation on the development process is scavengeless development. In a scavengeless development system, toner is detached from the donor roll by applying an AC electric field to self-spaced electrode structures, commonly in the form of wires positioned in the nip between a donor roll and a photoreceptor. This forms a toner powder cloud in the nip and the latent image attracts toner from the powder cloud. Because there is no physical contact between the development apparatus and the photoreceptor, scavengeless development is useful for devices in which different types of toner are supplied onto the same photoreceptor, such as in tri-level, recharge, expose and develop, highlight, or image-on-image digital color printing.
Since 1995, there have been many advances in high speed digital color printing technologies. The advances in marking technologies, from ink jet to xerography, with dry and liquid toners, have resulted in a diversity of products, each having speed and print qualities suitable for specific markets, such as small office or home office, general office, production printing, proofing and photo-finishing machines. At the same time, advances in microprocessors have enabled image processing and process control in digital color printing machines. These advances have increased productivity, image quality, substrate latitude, and run cost.
Digital color printing technology is still evolving to improve productivity, image quality, substrate latitude, and run cost. Each emerging technology has its own niches and barriers. For example, ink jet printing is architecturally simple, but presents challenges in the design of a quick drying, moisture resistant ink and robust page-wide ink heads for high speed printing on a wide selection of substrates at a reasonable run cost.
Three types of tandem architectures for electrophotographic or xerographic printing have emerged that vary primarily in where the color image is built (i.e., constructed or accumulated). The color image separations may be built on (1) paper, (2) an intermediate belt or drum or (3) a photoreceptor. The term tandem is used herein to refer to architectures where the color image separations are built either on paper or on an intermediate belt or drum in contrast to tandem architectures, the term image-on-image (IOI) is used herein to refer to architectures where the color image separations are built on a photoreceptor, and then transferred directly to a substrate (e.g., paper) or to an intermediate belt or drum. One fundamental difference between the tandem architecture and the image-on-image architecture is where the color image is built.
The color images produced by image-on-image or tandem digital color printing machines are typically four color images. In the image-on-image architecture, the four color image is built on one photoreceptor and transferred in a single step to a substrate (e.g., a plain piece of paper). Building the color image on the photoreceptor includes placing different colors on top of as well as adjacent to each other. In the tandem architecture, the four color image is built either on paper or on an intermediate belt or dram. Each color is transferred separately from one photoreceptor to the substrate, either directly to the substrate or through the intermediate belt or drum. Thus, another difference is throughput. Image-on-image architectures may apply multiple colors in a single transfer cycle, whereas tandem architectures require multiple cycles with one color being laid down during each cycle.
Digital color printing technology is still evolving and the performance of conventional digital color printing is currently limited by two problems. First, the color gamut is limited. Second, transfer subsystems sometimes cause lead and trail edge defects and wrinkle defects, especially for lightweight coated stock.
A color gamut is a range of producible colors. Different color reproduction techniques have different color capabilities or gamuts. For example, color transparency films have comparatively large gamuts, as do color monitors. The color gamut that can be produced using process inks of cyan, magenta, yellow and black (K)(CMYK) toners on paper is similar. This is why some colors that can be displayed on a color monitor, especially bright saturated colors, cannot be produced exactly by a digital color printing system or a printing press. When printed, colors that fall outside of the printer gamut are typically mapped to printable colors.
Transfer subsystems in a digital color printing system move sheets of media along a path inside the machine. The path a print job follows from creation to destination is called workflow. For example, a transfer subsystem may move sheets from input feeders or trays through various stations of the imaging process, including development, and then to output stacks.
Digital production presses have many applications, including short-run on demand printing of brochures, books, flyers, postcards, newsletters, catalogs, manuals, point of purchase materials and sell sheets. Various kinds of stock may be used in such applications, including coated, uncoated, textured, smooth and specialty stock. Of these, lightweight coated stock may be used to print textbooks on production equipment for digital color printing, such as a digital production press. Stock is sheets of media, such as paper.
Exemplary embodiments include a xerographic printing machine including a photoreceptor, a first and second set of different development housings, a biased transfer belt, and a fuser. The first set of development housings are arranged in an image-on-image configuration in proximity to the photoreceptor. The biased transfer belt is in proximity to the photoreceptor at a transfer station. The second set of development housings are arranged in a tandem configuration in proximity to the biased transfer belt. The fuser is in proximity to the biased transfer belt.
In exemplary embodiments, the first and second development housings may include color toners. One or more of the first or second set of development housings may perform semi-conductive magnetic brush development. In a particular embodiment, there are four development housing in the first set, each housing including one of four different color toners, while the second set of development housings includes at least two additional color toners.
The xerographic printing machine may also include biased transfer rollers made from, for example, at least one of the following materials: metal, rubber, polyamid, elastomer, or foam. The biased transfer rollers may be sized to allow a substrate to self-strip from the surface of the biased transfer belt. In exemplary embodiments, the biased transfer belt may be entrained around the biased transfer rollers and adapted to rotate. Additionally, the biased transfer belt may have a plurality of layers and/or a back coating.